Stimulation of glucose oxidation by liver homogenates with hexose 6-phosphates

Abstract

Liver homogenates incubated with labeled glucose yield increasing values of 14CO 2 as the concentration either of D-glucose 6-phosphate, 2-deoxy- D-glucose 6-phosphate, or D-glucosamine 6-phosphate is increased. The maximal stimulation of 14CO 2 production with increasing hexose phosphate concentration comes from systems with added NADP +; added NAD + does not significantly change values of expired 14CO 2 relative to the controls, while added ATP caused dilution in the CO 2 labeling. 2-Deoxy-glucose 6-phosphate was the most effective of the hexose phosphates stimulating 14CO 2 production, followed by glucosamine 6-phosphate. The former was found to stimulate the production of an unidentified lactone, which, however, was not 6-phosphogluconolactone. 6-Phosphogluconate caused some 14CO 2 increase, but this was less than that due to glucose 6-phosphate. None of the non-phosphorylated compounds tested, such as 2-deoxy- D-glucose, causes the stimulation of 14CO 2 production, nor does inorganic phosphate or pyrophosphate. D-Gluconate addition results in a slight dilution of the CO 2 labeling. Time studies on the rate of 14CO 2 production indicate that the phosphate esters themselves are the cause of the increased 14 CO 2 values. [I- 14C]glucose, when used as substrate, gives about twice the value of expired 14CO 2 relative to control as [6- 14C]glucose in the presence of increasing concentrations of of glucose 6-phosphate, glucosamine 6-phosphate, or 2-deoxyglucose 6-phosphate. The stimulation of 14CO 2 production by 2-deoxyglucose 6-phosphate in the presence of NADP + was also found using kidney homogenates. Stimulation was not observed using homogenates prepared from heart, spleen or brain. The enzyme, glucose dehydrogenase (β- D-glucose:NAD(P) oxidoreductase, EC i.i.i.47), has been found in kidney and, previously, in hepatic microsomes. No glucose dehydrogenase activity could be demonstrated in preparations from brain, spleen and heart. The results obtained are the opposite to those predicted on the basis of isotopic dilution by a metabolic intermediate, in addition to those predicted by enzyme inhibition caused by the phosphate esters employed in these experiments. This suggests the involvement of a pathway of glucose oxidation which does not involve glucose 6-phosphate as an intermediate. This pathway, under the conditions employed, is stimulated by NADP + and certain hexose phosphates and inhibited by ATP.